This invention is directed to controllable RF power generators and is particularly concerned with control circuitry permitting the RF energy to be generated at a predetermined frequency over a range of power levels.
RF power generators are often used in industrial or manufacturing processes, for example, to provide RF energy to generate a plasma for sputter coating or etching applications in the semiconductor industry. In many cases, the applied power must be controlled rather precisely for accurate control of deposition rate.
In the art of deposition or sputtering, for example, the process is driven by radio frequency energy typically provided at a frequency of 13.56 MHz at levels from several watts to several kilowatts. Typically there is an RF generator coupled to the plasma chamber through a matching network. A typical RF power generator employs an analog control loop to achieve fast response with reasonable accuracy over a major portion of the output power range. Most often, the output power (forward and reflected) is metered using a VSWR bridge or directional coupler to form a feedback voltage which is then fed through a squaring circuit to provide a signal that is proportional to the output power. This is then balanced against a control voltage that corresponds to a demand power level. The gain of the power generator is adjusted until the measured power level and the power demand level are in balance. However, because of inaccuracies in the VSWR bridge circuit for measuring output power, especially at lower power levels, the accuracy can be unacceptably low. Thus, it has long been desired to provide an improved feedback system wherein the RF output power is controlled so as to respond linearly to the DC power demand or control voltage.
It has been proposed to employ digital techniques to achieve high accuracy. With a digital system it is possible to compensate for the non-linear characteristic of the metering circuit. However, a digital system tends to be rather slow, and requires a number of cycles to follow any significant changes in the control voltage. On the other hand, a high-speed digital system tends to be rather expensive and complex, and difficult to engineer. Thus, while a digital feedback system would appear to be ideal, its high cost and complexity would tend to disqualify it as a suitable alternative.
It has not been possible in this art to obtain a greater level of accuracy with analog feedback, and provide high accuracy at reasonable costs and without sacrificing the high-speed correction characteristics of analog control.